1. Field of the Invention
The illustrative embodiments of the invention relate generally to a pump for pumping fluid and, more specifically, to a pump having a substantially disc-shaped cavity with substantially circular end walls and a side wall and a valve for controlling the flow of fluid through the pump in conjunction with an electronic circuit for driving a square-wave signal that reduces harmonic excitation of the pump.
2. Description of Related Art
The generation of high amplitude pressure oscillations in closed cavities has received significant attention in the fields of thermo-acoustics and pump type compressors. Recent developments in non-linear acoustics have allowed the generation of pressure waves with higher amplitudes than previously thought possible.
It is known to use acoustic resonance to achieve fluid pumping from defined inlets and outlets. This can be achieved using a cylindrical cavity with an acoustic driver at one end, which drives an acoustic standing wave. In such a cylindrical cavity, the acoustic pressure wave has limited amplitude. Varying cross-section cavities, such as cone, horn-cone, bulb have been used to achieve high amplitude pressure oscillations thereby significantly increasing the pumping effect. In such high amplitude waves the non-linear mechanisms with energy dissipation have been suppressed. However, high amplitude acoustic resonance has not been employed within disc-shaped cavities in which radial pressure oscillations are excited until recently. International Patent Application No. PCT/GB2006/001487, published as WO 2006/111775 (the '487 Application) discloses a pump having a substantially disc-shaped cavity with a high aspect ratio, i.e., the ratio of the radius of the cavity to the height of the cavity.
Such a pump has a substantially cylindrical cavity comprising a side wall closed at each end by end walls. The pump also comprises an actuator that drives either one of the end walls to oscillate in a direction substantially perpendicular to the surface of the driven end wall. The spatial profile of the motion of the driven end wall is described as being matched to the spatial profile of the fluid pressure oscillations within the cavity, a state described herein as mode-matching. When the pump is mode-matched, work done by the actuator on the fluid in the cavity adds constructively across the driven end wall surface, thereby enhancing the amplitude of the pressure oscillation in the cavity and delivering high pump efficiency. The efficiency of a mode-matched pump is dependent upon the interface between the driven end wall and the side wall. It is desirable to maintain the efficiency of such pump by structuring the interface so that it does not decrease or dampen the motion of the driven end wall thereby mitigating any reduction in the amplitude of the fluid pressure oscillations within the cavity.
The actuator of the pump described above causes an oscillatory motion of the driven end wall (“displacement oscillations”) in a direction substantially perpendicular to the end wall or substantially parallel to the longitudinal axis of the cylindrical cavity, referred to hereinafter as “axial oscillations” of the driven end wall within the cavity. The axial oscillations of the driven end wall generate substantially proportional “pressure oscillations” of fluid within the cavity creating a radial pressure distribution approximating that of a Bessel function of the first kind as described in the '487 Application which is incorporated by reference herein, such oscillations referred to hereinafter as “radial oscillations” of the fluid pressure within the cavity. A portion of the driven end wall between the actuator and the side wall provides an interface with the side wall of the pump that decreases dampening of the displacement oscillations to mitigate any reduction of the pressure oscillations within the cavity, that portion being referred to hereinafter as an “isolator.” The illustrative embodiments of the isolator are operatively associated with the peripheral portion of the driven end wall to reduce dampening of the displacement oscillations.
More specifically, the pump comprises a pump body having a substantially cylindrical shape defining a cavity formed by a side wall closed at both ends by substantially circular end walls, at least one of the end walls being a driven end wall having a central portion and a peripheral portion adjacent the side wall, wherein the cavity contains a fluid when in use. The pump further comprises an actuator operatively associated with the central portion of the driven end wall to cause an oscillatory motion of the driven end wall in a direction substantially perpendicular thereto with a maximum amplitude at about the centre of the driven end wall, thereby generating displacement oscillations of the driven end wall when in use. The pump further comprises an isolator operatively associated with the peripheral portion of the driven end wall to reduce dampening of the displacement oscillations caused by the end wall's connection to the side wall of the cavity as described more specifically in U.S. patent application Ser. No. 12/477,594 which is incorporated by reference herein. The pump further comprises a first aperture disposed at about the centre of one of the end walls, and a second aperture disposed at any other location in the pump body, whereby the displacement oscillations generate radial oscillations of fluid pressure within the cavity of said pump body causing fluid flow through said apertures.
Such pumps also require one or more valves for controlling the flow of fluid through the pump and, more specifically, valves being capable of operating at high frequencies. Conventional valves typically operate at lower frequencies below 500 Hz for a variety of applications. For example, many conventional compressors typically operate at 50 or 60 Hz. Linear resonance compressors known in the art operate between 150 and 350 Hz. However, many portable electronic devices including medical devices require pumps for delivering a positive pressure or providing a vacuum that are relatively small in size and it is advantageous for such pumps to be inaudible in operation so as to provide discrete operation. To achieve these objectives, such pumps must operate at very high frequencies requiring valves capable of operating at about 20 kHz and higher. To operate at these high frequencies, the valve must be responsive to a high frequency oscillating pressure that can be rectified to create a net flow of fluid through the pump.
Such a valve is described more specifically in International Patent Application No. PCT/GB2009/050614 which is incorporated by reference herein. Valves may be disposed in either the first or second aperture, or both apertures, for controlling the flow of fluid through the pump. Each valve comprises a first plate having apertures extending generally perpendicular therethrough and a second plate also having apertures extending generally perpendicular therethrough, wherein the apertures of the second plate are substantially offset from the apertures of the first plate. The valve further comprises a sidewall disposed between the first and second plate, wherein the sidewall is closed around the perimeter of the first and second plates to form a cavity between the first and second plates in fluid communication with the apertures of the first and second plates. The valve further comprises a flap disposed and moveable between the first and second plates, wherein the flap has apertures substantially offset from the apertures of the first plate and substantially aligned with the apertures of the second plate. The flap is motivated between the first and second plates in response to a change in direction of the differential pressure of the fluid across the valve.
The actuator may be a piezoelectric actuator that resonates at multiple frequencies in addition to its fundamental frequency, the frequency at which the actuator is intended to be driven. Piezoelectric drive circuits typically employ square-wave drive signals for such actuators because the drive circuit electronics may be lower cost and more compact. These factors are important, for example, in medical devices that may be used to generate a reduced pressure for treating wounds, and in other applications where a compact pump and drive electronics are required. A problem encountered when utilizing a square-wave as the drive signal for such actuators is that a square wave contains additional frequencies at multiples of its fundamental frequency (f), i.e., harmonic frequencies, that can coincide with, or be sufficiently close to, higher-frequency resonant frequencies of the actuator associated with other oscillatory modes (e.g. higher order “bending” modes or radial “breathing” modes of the actuator) that are excited along with the actuator's fundamental mode. Excitation of these modes may substantially reduce the performance of the actuator and, consequently, the pump. For example, excitation of such higher frequency modes may lead to increased power consumption resulting in reduced pump efficiency.